Substrates are coated for a wide variety of reasons. Coatings may be applied to protect the underlying substrate from weathering effects and/or to provide a pleasing visual appearance. Dual-purpose coatings are particularly common in the automotive industry where excellent weatherability, durability and appearance are minimum requirements for original equipment coatings applied in the assembly plant by the automotive manufacturer. Many types of coatings and coating systems have been developed in an attempt to satisfy the requirements of the automotive industry.
Composite color-plus-clear coatings are widely utilized in the coatings art. They are particularly desirable where exceptional gloss, depth of color, distinctness of image, or special metallic effects are required. The automotive industry has made extensive use of color-plus-clear composite coatings for automotive body panels.
As used herein, the term "composite color-plus-clear" relates to composite coating systems requiring the application of a first coating, typically a colored basecoat coating, followed by the application of a second coating, generally a clearcoat, over the noncured or "wet" first coating. The applied first and second coatings are then cured. Thus, such systems are often described as "wet on wet" or "two-coat/one bake". Drying processes that fall short of complete cure may be used between the application of the coatings.
Clearcoats used in color-plus-clear systems must have an extremely high degree of clarity in order to achieve the desired visual effects. High gloss coatings also require a low degree of visual aberrations at the surface in order to achieve the desired visual effect such as high distinctness of image (DOI). As a result, clearcoats of color-plus-clear systems are especially susceptible to the phenomenon known as environmental etch, i.e., spots or marks on or in the clear finish that often cannot be rubbed out.
Many compositions have been proposed for use as the clearcoat of a color-plus-clear composite coating. However, prior art clearcoat coatings often suffer from disadvantages such as inferior coatability, compatibility problems with the colored basecoat, solubility problems, lack of adhesion to the underlying basecoat, lack of adhesion to repair coatings applied to the color-plus-clear composite coating subsequent to the cure of the clearcoat, and/or insufficient resistance to environmental etch.
Clearcoat coating compositions exhibiting many advantages over prior art compositions, especially with respect to environmental etch, were disclosed in U.S. Pat. Nos. 5,474,811 and 5,356,669. The disclosed clear coating compositions comprise a first component comprising a polymer backbone having appended thereto at least one carbamate functional group, and a second component comprising a compound having a plurality of functional groups that are reactive with said carbamate groups.
However, despite the foregoing improvements, certain of the aforementioned problems remain. In particular, some composite coatings exhibit an inferior degree of adhesion to repair coatings applied subsequent to cure of the composite coating. Inferior or inadequate adhesion of repair coatings to the composite coating is a separate and distinct problem from delamination of the overall composite coating to the underlying substrate.
The type of repair coating and method used to repair a defect in a finished surface typically depends upon when and how the defect occurred.
Defects and/or imperfections that occur during the application and/or cure of the composite coating are corrected via "in-line" repair processes. In-line repair processes normally involve the application of one or more additional layers of the composite coating to the surface of one or more previously applied and cured layers of the composite coating. The additional layers are cured in a manner identical to the previously applied and cured layers. Multiple passes of the defect containing cured-coated product through the composite coating application line and oven system accomplish this type of repair. It will be appreciated that the repair coating in this instance is the composite coating itself or the components thereof. Repair may be done on the basecoat component, the topcoat component, or the combination thereof, i.e., the color-plus-clear composite coating. In any case, in-line repair is done with the application of a coating that is identical to the coating layer or layers to be repaired. No sanding or surface preparation is permitted prior to the application of the additonal composite coating layers or components thereof. The "repair coating", i.e., the composite coating, must exhibit excellent adhesion to the previously applied and cured composite coating layers without any sanding or preparation of the previously applied and cured layers.
Defects and/or imperfections which result from product manufacturing processes are addressed with "end of line" repair processes. End of line repair processes typically occur after the addition of product components having melting and/or deformation temperatures lower than the bake temperature of the color-plus-clear composite coating. Thus, a repair coating having a cure temperature lower than the lowest component deformation temperature is employed. As a result, end of line repair coatings have cure temperatures significantly lower than those of composite coatings. Some sanding and/or surface preparation may be allowed prior to the application of the end of line repair coating.
Finally, "refinish" repair processes are used to rectify defects that occur after the product leaves the manufacturing facility. Refinish products are typically applied after substantial sanding of the surface to be repaired. Refinish coatings typically cure at temperatures less than those of color-plus-clear composite coatings but are designed to provide optimum performance benefits. Such coatings may be expensive and require specialized application equipment.
Manufacturers of composite coatings are particularly concerned with the requirements of in-line repair processes. Improvements in in-line repair coating adhesion must be provided in conjunction with required performance properties of the composite coating such as appearance, durability, and etch resistance among others.
It is desirable to provide a method to improve the adhesion of repair coatings applied to a previously coated substrate in an in-line repair process. Such a method would provide quality, appearance and cost advantages. It would also be advantageous to provide a composite coating which provides desirable performance properties but is also capable of performing as a repair coating for use in in-line repair processes without sanding or surface preparation of the defect-containing coated substrate.
The present invention achieves these and other objectives. The instant invention is directed toward a method to improve the adhesion of a repair coating applied to a previously coated substrate in an in-line repair process. The invention further provides coating compositions for use in the claimed method and coated articles resulting therefrom are also provided.
The method of the invention for improving the adhesion of a repair coating to a coated substrate requires that at least one coating composition be applied to a substrate wherein the coating composition comprises (a) a film forming component comprising (i) a crosslinking component comprising one or more aminoplast resins, and (ii) one or more polymers having one or more functional groups which are reactive with the crosslinking component, (b) an adhesion improving additive comprising one or more compounds containing Sn(IV), (c) a catalyst for promoting a reaction between crosslinking agent (i) and one or more polymers (ii). The at least one coating composition is cured to provide a coated substrate and one or more repair coatings are applied to the coated substrate. In a preferred aspect of the invention, the one or more repair coatings applied are the same as the at least one coating composition.
The coating composition of the invention having improved adhesion to repair coatings requires (a) a film-forming component consisting of (i) a crosslinking component consisting of one or more aminoplast resins, and (ii) one or more polymers having one or more functional groups which are reactive with the one or more aminoplast resins, (b) an adhesion improving additive comprising one or more compounds containing Sn(IV), and (c) a catalyst for promoting a reaction between the one or more aminoplast resins (i) and one or more polymers (ii), the catalyst comprising one or more acidic compounds which are at least partially blocked. In a preferred aspect of the invention the composition further comprises (d) an adhesion promoting component consisting of (i) an epoxy group containing component, and (ii) a monofunctional carboxy group component.
Finally the invention provides a coated article comprising an article having a substrate with a cured coating thereon, the cured coating comprising the result of curing the at least one coating composition of the invention.